In some semiconductor devices, a bus access detection circuit in a power management bridge (PMB) circuit has a power supply control function to a slave circuit as a partial circuit in the semiconductor device for the purpose of reducing power consumption while the circuit is not used.
In such a semiconductor device, when bus access to a slave circuit is issued while the slave circuit is off, the bus access detection circuit in the PMB circuit makes power-on notification to a power switch (PSW) control circuit.
Upon receiving the power-on notification, the PSW control circuit sends a power switch control (PSC) signal to the PSW circuit to turn on the slave circuit. In addition, the PMB circuit causes access from the bus to wait until the slave circuit is turned on and after a certain period of time during which the slave circuit is turned on and the power supply is stabilized passes, the access from the bus is conveyed to the slave side.
However, if it takes time until the slave circuit is turned on, it will take time before the access is completed. Therefore, when on/off frequently occurs in the power supply control, it takes time to access each time the slave circuit is turned on in an off state.